1. Field
This disclosure is concerned generally with filters and specifically with filters for human blood.
2. Prior art
When human red blood cells (RBC's) are transfused to a patient, they are first routinely passed through a filter known as a blood filter (or RBC filter) to remove blood clots or aggregates larger than a given size, usually expressed in microns. The majority of blood filters in use today are designed to filter out all clots and aggregates larger than about 210, 150 or 80 microns. These can be referred to as macroaggregate filters. However, there is a growing need and desire to filter out aggregates larger than about 40 microns. The filters that are capable of filtering out 40 micron and larger aggregates are commonly referred to as microaggregate filters. The present disclosure is concerned with an improved microaggregate filter (i.e. a blood filter capable of filtering clots and aggregates larger than about 40 microns from one or, preferably more, units of blood or red blood cells, prior to administration to a patient).
Present microaggregate filters are available in a variety of designs. Such filters may use either multiple layers of filter media or a relatively large area (&gt;135 square centimeter) of a single filter of about 20 to 40 micron pore size. The latter design is commonly folded in accordion-like fashion to reduce the product to a size more convenient to use and store. In at least one such filter, the accordion pleats are separated by a relatively large mesh separation screen (1,000 micron pore size) which does not act as a filter medium. Microaggregate filters having such designs are described below and illustrated in the first two figures under the heading, prior art as FIGS. 1 and 2.
The above prior art designs are often unsatisfactory, especially for use in filtering more than a single unit of RBC's. Filters similar to that illustrated by FIG. 1, while effective for small quantities of blood, tend to become blocked by accumulated microaggregates and gelatinous clots, thereby forming occlusions at the filter surface of initial contact by the blood. The relatively small filtering surface area contributes to this problem. The filters of FIG. 2, while attempting to increase the filtering surface area per unit volume with pleats, tend to quickly occlude because the narrow pleat openings (typically about 1 mm) and shallow channels (typically about 1.5 cm) fill with clots, thereby rendering the larger surface area relatively useless. The large mesh (1,000 micron) screen separating the pleats is not designed to retain clots or aggregates of the type described in this disclosure.
A relatively recent attempt to avoid the above problems has been described in an Abstract at page 144 of the 1984 Proceedings of the International Society of Blood Transfusion. The Abstract by W. H. Walker et al. is entitled, "A Simple Transfusion Device for Filtration of Blood with a 40 .mu.m Filter Screen." The filtration system consists of two filter screens with 150 and 40 micron mesh sizes and filter surface-areas of 23 and 36.5 cm.sup.2, respectively. The device is described as having a priming volume of 35 ml and is said to be useful for both gravity and pressure transfusion. Experiments were said to show that 500 ml of CPD whole blood stored three weeks could be filtered by gravity flow within 2-3 minutes with a unit of packed CPD RBC's taking 3-4 minutes. Although the Abstract does not describe further filter structure details, the filter appears to have a relatively small volume to surface area ratio, thus raising questions as to whether, with time or with multiple units of blood being filtered, the accumulation of larger (&gt;150 micron) clots and aggregates will permit an unimpeded flow of blood through the smaller micron (40) screen.
After considering the advantages and disadvantages of existing microaggregate filters, we have devised a significantly improved microaggregate filter which addresses the problems of flow obstruction associated with existing microaggregate filters. Details of our new filter are described below.